The major focus of this project is the development of a vaccine to prevent or control infections caused by Chlamydia trachomatis. The chlamydial major outer membrane protein (MOMP) is considered to be important in the development of protective immunity to chlamydial infection. Thus, current vaccine strategies are focused on the development of recombinant or synthetic peptide MOMP vaccines that can be target to induce mucosal immunity. We have undertaken an extensive effort aimed at characterizing the antigenic properties of the MOMP at the molecular level to identify immunologically relevant structures of this protein that may have utility for the rational design of a chlamydial vaccine. Previous efforts were focused on characterizing immunodominant neutralizing B-cell epitopes of the MOMP. Serovar-specific and broadly cross reactive subspecies-specific neutralizing sites were mapped to contiguous epitopes located in within the variable domains (VD) of the protein. In this year, we have undertaken studies to characterize T-helper cell antigenic determinants of the MOMP that function in inducing B-cells to produce antibody specific to neutralizing B-cell epitopes. Two regions of the MOMP were shown to contain Th-cell epitopes capable of directing B-cell clones to produce antibody specific to immunodominant neutralizing sites. Chimeric peptides containing both Th-and B-cell epitopes were synthesized and shown to be highly immunogenic. Immunization of mice with the chimeric peptides induced high titered antibodies that reacted with the native MOMP and were neutralizing in vitro. The Th-cell epitopes under study are apparently "promiscuous" since strains of congeneic mice differing at H-2 are capable of responding to the chimeric peptide immunogens. Evaluation of the immunogenicity and vaccine efficacy of the chimeric peptides in sub-human primates will be initiated in the upcoming year. We are also working on the construction of infectious vectored chlamydial vaccines. Towards these goals we have expressed the complete MOMP protein in vaccinia virus and have evaluated the immunogenicity of the recombinant virus in mice. The results are promising; mice infected with recombinant vaccinia virus expressing the MOMP induced high titered antibodies specific to the protein. We plan to evaluate the protective efficacy of the recombinant virus in animal models of chlamydial infection in the upcoming year.